The present invention involves the ejection of ink drops by way of forming gas or vapor bubbles in a bubble forming liquid. This principle is generally described in U.S. Pat. No. 3,747,120 (Stemme). Each pixel in the printed image is derived ink drops ejected from one or more ink nozzles. In recent years, inkjet printing has become increasing popular primarily due to its inexpensive and versatile nature. Many different aspects and techniques for inkjet printing are described in detail in the above cross referenced documents.
Completely immersing the heater element in ink dramatically improves the printhead efficiency. Much less heat dissipates into the underlying wafer substrate so more of the input energy is used to generate the bubble that ejects the ink.
To immerse the heater, it needs to be spaced from the floor of the ink chamber and so the heater material is usually deposited on a layer of sacrificial material (SAC) that is subsequently removed by a release etch. The contacts at either end of the heater element are raised to the height of the SAC layer by some type of permanent scaffolding structure. The contacts need vertical or inclined portions to establish an electrical connection between the electrode areas on the top metal layer of the CMOS drive circuitry and the heater element. However, heater material deposited on vertical or inclined surfaces is thinner than on horizontal surfaces. To avoid undesirable resistive losses from the thinner sections, the contact portion of the thermal actuator needs to be relatively large. Larger contacts occupy a significant area of the wafer surface and limit the nozzle packing density.